The present invention relates to a vehicle wheel.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The conception of modern wheel rims is subject to various, partially conflicting, demands. Beside a simple wheel rim contour which enables an easy manufacture, a central demand is that the wheel rim allows a maximal air flow through the wheel rim to be able to optimally cool the wheel rim by the air flow if needed. Further, during a residual heating period of the brake disc at standstill, it has to be ensured that the heat can be dissipated also without air flow. This results in the demand for a maximally “open” wheel rim structure.
However, for aerodynamic reasons the wheel rim should be maximally closed, to prevent air turbulences as far as possible, which are generated by openings in the wheel rim. During normal driving, an overpressure exists in the region of the underside of the vehicle, which ensures that air is pushed through the free spaces of the wheel rim. If the wheel rim was configured as closed disc, the cw-value which is included in the calculation of the air resistance of the vehicle could be improved.
In order to meet these requirements which quasi contradict one another on one hand for thermal reasons and on the other hand for aerodynamic reasons, it is known to be able to open and close the wheel rim side openings depending on the need. In such systems, the plate shaped cover elements are usually pivotally supported and movable radially along the pivot axis. The cover elements are each additionally connected to the wheel rim via a force guiding coupling element. The centrifugal force acting on the cover elements increases with increasing rotational speed of the wheel which causes the cover elements to move outwardly starting from a radially inner position in which they open the respectively assigned opening, wherein during this radial movement which is due to centrifugal force, the cover elements pivot from the open position into the closed position as a result of the forced guidance via the coupling element. This radial movement occurs against the force of a restoring element, usually a spring which urges the respective cover element radially inwards again which, however, due to the high centrifugal forces is overcome and pushed back. When the rotational speed of the wheel decreases again the movement is inverted. The cover elements open automatically via the force guiding coupling element when the decreasing centrifugal forces allow the restoring element to cause the cover elements to move radially inward, and to assume their inner position.
EP 0 145 487 A2 discloses a temperature controlled actuating element, for example in the form of a memory alloy or a thermal bimetal, which deforms when reaching a sufficiently high actuating temperature and thereby exerts a force on the assigned cover element in order to urge the latter from the radially outward position back into the radially inward open position. This mechanism has the purpose to enable cooling via an air flow through the wheel rim when, in spite of high driving speeds, the temperature in the region of the brake is sufficiently high to absolutely require the cooling. Here, the high centrifugal force has to be overcome, to push the cover element back again via the force guiding coupling element. This places high demands on the respective thermally controlled actuation elements, which thus have to have the corresponding working capacity.
In principle, several distinguishable states result from the previously mentioned demands on an “active” vehicle wheel. When the vehicle is stationary, the plate shaped cover elements are open. On one hand, this creates a 3D-Effect (depth perception) which is often desired for design reasons, on the other hand the waste heat can be dissipated through the wheel rim, when the brake has residual heat as a result of a previous drive. Above a certain vehicle speed for example 30-40 km/h the design of the wheel can no longer be differentiated by the human eye, only a rotating disc is visible. Above this threshold speed the cover elements should close, to improve the aerodynamics. On the other hand, when the brake is operated during the drive, and the brake disc heats up, the cover elements have to open again in order to ensure aeration of the brake system. Thus, the following changes of states result with regard to the cover element:
Cover element open→cover element closed, when:
    v>vgrenz and T<Tgrenz Cover element closed→cover element open when:    v<vgrenz or T>Tgrenz 
wherein v means the actual speed, vgrenz the previously described threshold speed, T the actual temperature in the region of the vehicle brake, and Tgrenz a threshold temperature above which the cover elements are to be opened.
In known vehicle wheels with the previously described movable force guided cover elements, the rigidity or stiffness of the force guide via which the coupling element poses a problem however, for example when driving toward a curb during parking or in the case of vandalism. In both cases, the cover element is located in the pivoted out, open position, i.e. positioned in the wheel rim inner starting position. When the vehicle approaches the curb, it can occur that the cover element first abuts the curb with its pivoted section which projects sideways from the wheel rim plane. When proceeding with the vehicle movement, pressure may be exerted locally on a point of the cover element which is very close to the pivot axis, and thus a short lever exists and very high forces act on the bearing of the cover element, because the cover element does not follow the applied pressure due to the rigidity or stiffness of the force guiding. In the extreme case, damage to the cover element can result. Also in the case of vandalism, when pushing strongly against the projecting section of the cover element, damage can result.
It would therefore be desirable and advantageous to provide an improved vehicle wheel which allows closing the plate shaped cover elements also in case of negligible centrifugal forces by applying a limited force.